Sulfur gravel is mined here. A profitable business, it seems to me. Sulfuric acid can be extracted from it. I have also heard that zinc ore and barite are stored underground here. It's just as well that the days of horse-drawn vehicles will soon be history - after all, these new chemical plants are mostly further away on the Rhine.
The "Steel Time Travelers" Luise & Alfred: Mining Museum Siciliaschacht